To solve the problem that the plane area of a coil component increases to occupy a wide mounting area on a printed board, and it is difficult to lower the height of the entire structure of an electronic circuit such as DC-DC converters and miniaturize it.

A conductor pattern 22 is formed on one surface of a core 20. A coil component and a circuit component 40 mounted on the conductor pattern constitute a DC-DC converter circuit, etc., thereby thinning and miniaturizing a small electronic circuit device having the coil component. This device comprises a core 10, the flat plate-like core 20 having the conductor pattern 22 on the surface, a coil 30 wound around the core 10, the circuit component 40 connected to the conductor pattern 22 to mount on the core 20, and a plurality of terminals 50. An upper flange 11 of the core 10 is fitted into a hole 21 of the core 20 from the downside to adhere and fix the core 10 to the core 20, and both ends 31, 32 of the coil 30 are connected to the conductor pattern 22.